If you have ever looked out of the window of a large airplane as it approaches a runway for landing, you will have seen something called a “spoiler” in operation. Spoilers are plates on the top surface of a wing that can be extended upward into the airflow to “spoil” it. See FIG. 1-1. The spoiler creates a carefully controlled stall over the portion of the wing around it, greatly reducing the lift of that wing section. As the wing's lift decreases, the aircraft will assume a trajectory angle characteristic of the approach phase up to landing.
Depending on the landing conditions and other factors, it may be necessary or desirable to provide a steeper approach angle or trajectory. FIG. 1A contrasts example non-limiting conventional and steep approach procedures for an aircraft 10. Typical conventional approaches use a trajectory angle of around −3.3 degrees. In the FIG. 1A example, the trajectory angle for a steep approach is much larger—for example, on the order of −5.5 degrees (mathematically, 0°>Θappr>Θsteep appr.).
It is well known in aeronautical engineering that drag devices such as flight spoilers are capable of increasing the maximum descent ratio by increasing the trajectory angle (considering the absolute value of the trajectory angle). However, using the spoilers in this way can sometimes have drawbacks due to undesired side effects or risks.
In particular, for a given angle of attack, it is possible that as drag devices such as spoilers reduce the lift produced by the aircraft, they can also reduce significantly the margins to stall. Consequently, there is a concern regarding the speed margins that are required for safe flight operation. It is likely that speed increase will be necessary in order to regain the same stall margins as before, thus jeopardizing the performance of airplane on approaches and landings. This may result in deterioration of flying qualities during steep approaches and landings, including flare maneuvers. It could also result in exposure to high touch down sink rate and tail strike in steep approaches and landings.
Consider a steep approach operation with two possible glide slopes: a normal steep approach and an “abusive” steep approach (i.e., steeper than the normal steep approach). It is well known that aircraft have a maximum descent capability that is possible in a constant airspeed state flight. The maximum descent capability will, generally speaking, depend on the airplane's weight and on the lift, drag and engine's thrust produced by the airplane in a given flight condition.
Some operations require capability to perform a steady flight in a steeper descent path. Depending on the conditions mentioned above, a given airplane may lack this capability, especially in low airspeed conditions (e.g., those airspeed values commonly used to approach and landing). In these cases, the most commonly used solution is to introduce devices, mechanisms and methods to produce even more drag on the airplane utilizing the flight spoilers and/or other drag devices such as airbrakes, dive brakes and others, in order to allow a steeper descent trajectory.
Since there is a tradeoff between lift and drag, some attempts have been made in the past to automatically control the spoilers or other drag control surfaces, to reduce some of the above-mentioned effects. In some such prior approaches, the spoiler is biased to a predetermined position and a control feedback circuit is used to command the spoiler around the predetermined bias position. This predetermined bias position permits the aircraft to perform a steep approach landing in a predetermined flight envelope which includes different weight configurations. This bias position is typically dimensioned to cover the range of aircraft weight inside the steep approach flight envelope. In addition, spoiler bias position normally requires an increase in the approach airspeed in order to preserve or maintain sufficient margin above stall.
Heavier aircraft configurations require less drag to perform steep approach and landing than lighter configurations. If the bias position is the same for both, in the heavy weight configuration increasing the airspeed will lead to a long landing distance, frequently creating a limitation in maximum landing weight. In addition, this bias position is commonly designed to provide capability to increase the glide slope approach (beyond the Steep Approach target), allowing correction during the approach to landing. This means a higher biased spoiler position, which will also likely end up in a landing weight limitation.
To overcome these problems and provide improvements to aircraft safety, the example non-limiting technology herein adds a function that sets the biased position of the drag device(s) (spoilers, for example) depending on factors such as:
(1) the airplane estimated weight (for higher weight, the biased position will be small) so that the maximum landing weight limitation is decreased; and
(2) the measured flight path angle so that during most of the approach the drag device biased position will be small but, if flight path corrections (i.e., flight path increase) is needed, the drag device will be automatically offset to a higher biased position, ensuring satisfactory descent capability without introducing a maximum landing weight limitation.
Alternatively or additionally, the example non-limiting technology herein is capable of modulating the control position of drag devices as a function of pilot longitudinal commands in order to enhance glide slope control during approach, flare and landing. Also alternatively or additionally, the example non-limiting technology herein is capable of modulating the control position of drag devices as a function of only one of the three factors mentioned above (the airplane estimated weight, measured flight path angle or pilot longitudinal command) or as a function of any combination among these three factors (which may be a combination between two or among three of them). In other words, alternatively or additionally, the example non-limiting technology herein is capable of defining a function of drag device deployment as a function of only one of the three factors among the aircraft mass, trajectory angle and pilot inceptor position and/or movement or as a function of any combination among these three factors which may be a combination between two or among three of them.
The example non-limiting technology herein also provides for automatically retracting of the drag devices, if the angle of attack reaches a predefined threshold, in order to keep unchanged stall speeds and maneuvering capability.
The example non-limiting technology herein enables a steep approach and landing with a higher airplane weight. One benefit allows operation with a higher payload (e.g., more passengers). This means that fewer airplanes are needed to carry a given payload, which in a final analysis, can reduce air traffic and consequently fuel emissions.
In addition, performing an approach and landing with a steeper approach glide slope (i.e., the kind of approach which may be achieved by an airplane equipped with the example non-limiting technology herein) results in a reduction in the noise perceived in the areas surrounding the airport. Thus, the example non-limiting technology herein contributes positively to the environment in this additional way.
In more detail, an example non-limiting system and method may optimize the drag device positioning used to perform the Steep Approach to improve its performance. One example non-limiting embodiment provides means (e.g., probes) of measuring air data, means (e.g., AHRS) of measuring aircraft inertial information, means (e.g., inceptor) of sensing pilot input, means (e.g., Fly-By-Wire or avionics system) to process data and computing outputs to command the drag devices, means (e.g., hydraulic or electromechanical actuators) of actuating the drag devices and finally the drag devices itself (e.g., spoilers).
In one example non-limiting implementation, information obtained from the air data and the inertial system is fed back to estimate the current aircraft mass. This estimation is properly filtered to avoid high frequency content not pertinent to the aircraft's mass dynamics. The Steep Approach Logic receives the estimated aircraft mass together with the current aircraft trajectory angle and the desired trajectory angle (supplied by the pilot) to compute the optimum drag device position (drag device command) which is carried out by the drag device actuators.
As the aircraft mass is reduced, the drag devices are gradually repositioned to a position which provides suitable drag to perform steep approach.
Additionally to the modulation as a function of mass, the drag devices are also repositioned based on current trajectory angle and the trajectory angle desired by the pilot. For example, when the pilot intends to perform a steeper glide slope, after keeping the aircraft in the desired trajectory angle for a certain amount (period) of time, the drag devices are automatically repositioned to a position which provides suitable drag to perform steep approach with the desired trajectory angle. The opposite works in the same way: when performing a steep approach, if the pilot intends to perform a conventional approach (e.g., glide slope around 3 degrees), the drag devices will be automatically repositioned to provide suitable drag after a certain amount of time to reduce the drag.
The repositioning of the drag devices due to variations in the estimated mass, the current trajectory angle and the desired angle is, in one example non-limiting implementation, completely transparent to the pilot. The non-limiting system may adjust itself without any additional commands by the crew.
Using all the aforementioned mentioned techniques, the example non-limiting technology herein eliminates the need for an increased approach airspeed and consequently improves the aircraft maximum landing weight.